Kugelrohr

Ball tubes ( also ball tubes) are devices made ​​of glass, which - generally speaking - are used in the chemical laboratory, if it is necessary to bring the exclusion of air solids or liquids in contact with gases or vapors; Vapors can be condensed into bullet tubes. Glassblower ready to ball pipes by inflating a selective softening heated glass tube into a ball. The ball can be at the end or in the middle of the glass tube. It can also be bent into various shapes, such as U-shape.

History

Especially in the 19th century ball tubes were widely used in chemical laboratories. Therefore, the term " Kugelrohr " today in the English language commonly referred to as a loan word.

Putting a substance in a bulb tube, it can be heated in a flow of a gas. Reactive gases, such as ammonia, chlorine, hydrogen chloride, oxygen, hydrogen can then react with the substance. In the current of an inert gas (e.g. argon, nitrogen ) was heated solids and dried; sure there are better devices today. U-shaped ball tubes can be cooled in cold baths so that fluid accumulates in the ball.

Kugelrohr distillation

In the organic chemistry laboratory ball pipes serve for the distillative separation of small amounts of substances having a boiling point of less than 300 ° C. In general, the Kugelrohr distillation is carried out under vacuum in order to achieve a lower boiling point. Distillation in a Kugelrohr are appropriate if the quantity of substance is too low to be done with the devices for " simple distillation " (pistons, distillation bridge). As a rule, reaches for the ball tube, if you want to distill approximately 1 to 3 mL of substance, but can also substance volume less than 1 ml are distilled. Already at about 50 to 100 mg of distillate, the constitution and structure of the chemical compound can be determined (NMR spectrum, IR spectrum, mass spectrum).

A Kugelrohr distillation is particularly suitable for the isolation of substances from fractions of chromatographic separations and for the removal of volatile solvent residues. Many syntheses in the research laboratory are now carried out at a scale of 10 to 100 mg; a distillation in a Kugelrohr 's the end of the experiment.

Ball tubes are made of glass blowers in various sizes; they consist of at least two balls. The substance to be distilled, usually dissolved in a solvent is contacted with a long extended or long pipette hopper into the ball at the end of the tube, wherein the inside of the glass tube and the other balls is not wetted ( soiled ) may be. Before the distillation begins, the ( possible low boiling ) solvents, eg, Diethyl ether or dichloromethane removed by " applying a vacuum " while rotating the ball tube.

Image: Globe pipe (schematic).

For distillation, the end ball positioned in an " oven", whose temperature can be controlled. Once the boiling point has been reached, begins to the steam in the second ball located outside the furnace. This is cooled by sprinkling with water, touching it with ice or dry ice. The bulb tube has to be constantly rotated. This bumping be avoided, and the distillation material in the " Siedekugel " spread as a film on the inner wall; thus, the distillation of the thin film is carried out under mild conditions. One can classify the Kugelrohr distillation under the so-called short-path distillation.

Image: Diagram of a Kugelrohr distillation.

After completion of the distillation taking the bulb tube from the oven, allowed to cool to room temperature, and cut out with the aid of a glass cutter / diameter, the ball ( s) with the distillate. These scratches to the pipe between the balls around and breaks it down carefully.

With a curved pipette can be the distilled substance taken from the ball. The cut parts of the ball tube can be reassembled after thorough cleaning of the glass blower.

If there is a mixture of several volatile compounds, can to some extent a ' fractional ' distillation can be achieved by sliding the second, filled with the first distillate ball in the oven and now the third ball as a "template " cools. Depending on the number of balls, this game can be repeated. However, the effectiveness of the separation of substances with similar boiling points ( separation efficiency ) is quite low; in other words: the number of theoretical plates is small.

Construction

Originally, the ball tubes were rotated during distillation by hand back and forth. Later could take over with special gears, the rotary motion electric motors. The equipment needed for this were built in the workshops of the Chemical Institute. Generally available were Kugelrohr apparatus, after manufacturers had adopted their development of laboratory equipment.

Modern Kugelrohr apparatus has an electrically heated and thermostatically controlled oven in which the first ball is heated with hot air. The furnace is completed by a variable iris diaphragm behind the " Siedekugel " so that as little heat is lost. The glass tube inserted through a sealing ring ( O-ring) in a coupling, which is rotated by the controlled motor. A hose leading to the vacuum pump.

Image. Commercial Kugelrohr apparatus. ( 1) heating tube ( oven ) (2) container ( ball ) for the substance to be distilled ( 3) cooler with ice ( 4) electric motor for rotating the balls and the vacuum outlet. In the distillation of the furnace or the bulb tube is placed horizontally.

Instead of the "classic" ball tube, are excluded in the leaks in the vacuum, can be used for larger amounts of substance and small pistons that are put together by means of cuts. So the cuts are tight, but they need to be greased. This allows the distillate to be contaminated. The distillate is collected in a round bottom flask with two opposite sections, which sits on a shaft which is rotated in order to prevent superheating and to ensure a uniform distillation.

Image. Schematic of assembled from individual parts ball tube. Links: Siedekugel, middle: Ball for the distillate, Right: glass tube with ground glass.

Cooling is the distillate with either air, ice or dry ice.

Further use

A Kugelrohr oven can still be used for drying of solids, for sublimation or removal of solvents or residues thereof in a vacuum.